Selected Questions -Answers From All Experts Astronomy Forum (Radio Telescopes)

Question:  I am interested in the workings of radio telescopes. Can you explain for me how these instruments work?  -  Curious, Santa Fe, NM,

Answer:

Below I show the basic design of a dish type radio telescope, the one most often used:



Note the parts labeled, namely the  receiver, the parabolic dish and the antenna - essentially some element of metal wire or a metal rod - which can be said to be the heart of the instrument. The basic function is to capture incoming electromagnetic waves (in the radio spectrum) and turn it into an electric current. in the wire. (The wire for the antenna is usually made of a good conductor such as copper or aluminum).  What about the dimension of the antenna probe?  For maximum efficiency it should be one fourth of the size of the wave it's going to intercept. Thus, for the 21 cm line of hydrogen, the probe should be about 5 cm in length.

In this radio telescope sketch above I show a parabolic dish which has the receiver at the prime focus.  Note in the case of a parabolic dish that is has the property such that a radio waves reflected off of it all come to the focus at the same time.

Note the receiver basically amplifies the signal and passes it off to the detector where it can be measured then analyzed subsequently, e.g. using computers. All receivers are located at the focal point because even the best (antenna) wires experience loss due to thermal noise and so introduce a high  noise to signal ratio. So we want the amplification to occur as close to the antenna as possible.

Once the radio signal has been captured by the antenna and converted into a current then amplified, radio astronomers use a technique called "heterodyning" or mixing.  This allows them to change the frequency of the signal while preserving while preserving all the information it contains.


You can get an analogous idea to what goes on if you own an AM-FM radio and have ever tuned to a specific frequency. Thus, to tune to say 900 kHz on your radio dial this illustration helps to fix ideas:


In this case of prosaic radio reception - on the AM band, for example, "tuning" a station means moving a variable capacitor so it is in resonance with the incoming radio wave.  The tuner on your AM radio then acts as a bandpass filter. Here the bandwidth of filter = bandwidth of the signal (D f). 

For commercial AM this is generally  +  4.5 kHz.  We call 904.5 the "upper sideband" and 895.5 the lower sideband (i.e. f(c) + D f   or  f(c) -  D f). A similar process applies to the tuning of radio waves.   In the case of the prosaic radio we may use, heterodyning, which  means mixing the original EM radio signals down into the audio range (kilohertz) where our ears can pick them up.

In the case of the radio telescope  we use a superoheterodyne  receiver  which also features a center signal frequency (f(c)) that is amplified in a radio frequency (RF) amplifier, after which a mixer takes the weak signal and mixes it with a strong local oscillator signal at a frequency say f (o) producing an output signal at an intermediate frequency f' '. (Where f ' is directly proportional to the RF signal power).

One such radio source signal - after detection and analysis is shown below for Cygnus A at a frequency of 238.5 megahertz (MHz):

Now, every radio telescope receives waves that come from fairly restricted areas of  the sky and this is defined by what's called the beam.  Every antenna - and by extension radio telescope- has a main lobe called the "main beam" and also side lobes off to the side.  The side lobes have different causes but most result from diffraction (Consult the astronomy archives for past answers pertaining to diffraction in optical telescopes).   The pattern of a radio telescope beam and typical side lobes are shown below:

Note the side lobes are weak but can still represent a significant fraction of the power a radio telescope receives. Generally 80-90 percent of the signal entering a receiver from a typical dish comes through the main beam, the other 10-20 percent through the side lobes.  Now, in the case of the initial radio telescope diagram, try to imagine the wave fronts coming in at an oblique angle instead of directly., e.g.

Here, the arrival of the left side of the wave is later than the right, because it happens to travel an extra distance.  If one finds this distance d is such that:

 d sin  q     =     l/  2

Then we have the left -radio wave fully cancelling the right and no signal gets through. Obviously one wants to avoid this condition.  Ideally then, we need:  d sin  q     =  0.

Again, every radio telescope has maximum reception in location and this falls off according to the beam pattern.  A convenient way to specify this pattern is by using the beam width and one half the peak (power) height.   From this one can deduce the optimum beam width in terms of the resolution. Thus, if two objects are closer than the beam width then they will blur into one, and the astronomer will be unable to distinguish the sources.  This conveys the importance of a narrow beam width, which just means the signal falls off rapidly as one shifts from one side of the source to the other.The narrower the beam width the better the resolution of the radio telescope. In general:


Beam width =  Resolution =   Wavelength /   diameter    =     l/  D 

Hence, if the wavelength is 21 cm (say for the hydrogen line) and the diameter D  of the telescope is 10 m (1000 cm) we have:   21cm /  1000 cm  = 0.021 arcseconds

Again, this answer entails only the most basic introduction of how a radio telescope works. To get into more details it is good to consult basic texts such as this excellent introduction (see Chs. 4-6)